Fix Swift 6 @MainActor isolation error

[devxoul]

Summary

Fix the Swift 6 strict concurrency error when using then/with/do with @MainActor-isolated types such as UIKit classes.

Closes #106

Problem

In Swift 6 strict concurrency, calling then on a @MainActor-isolated type like UIStackView produces:

Converting function value of type '@mainactor (UIStackView) throws -> Void' to '(UIStackView) throws -> Void' loses global actor 'MainActor'

This happens because the library was compiled with swift-tools-version:5.0 (Swift 5 language mode), where non-escaping closures do not inherit the caller's actor isolation.

Changes

Package.swift

• Bump swift-tools-version from 5.0 to 6.0. In Swift 6 language mode, non-escaping closure parameters automatically inherit the caller's actor isolation — no API changes needed.
• Update minimum deployment targets to macOS 10.13 / iOS 12 / tvOS 12 / watchOS 4 (required by Swift 6 toolchain).

Tests

• Add 3 new @MainActor-annotated tests validating that then, with, and do compile and behave correctly with @MainActor-isolated types.

Context

• Zero API surface changes. Existing consumers on Swift 6 get the fix automatically.
• Consumers still on Swift 5.x should pin to a previous release, as the new swift-tools-version:6.0 requires Swift 6 tooling.

Testing

• All existing tests pass.
• 3 new tests (testThen_MainActorIsolated, testWith_MainActorIsolated, testDo_MainActorIsolated) verify the fix.

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Summary by cubic

Fixes Swift 6 strict concurrency errors when calling then/with/do on @MainActor-isolated types by building Then in Swift 6 mode. Also updates minimum platform versions, adds @MainActor tests, and sets Tuist SWIFT_VERSION to 6.0.

• Migration
  • Requires Swift 6 toolchain (swift-tools-version: 6.0).
  • Minimum targets: macOS 10.13, iOS 12, tvOS 12, watchOS 4.
  • If you need Swift 5.x, pin to a previous Then release.

^{Written for commit 3b2f791. Summary will update on new commits.}

[cubic-dev-ai]

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